Here we look at the traditional methods JavaScript has available for running code asychronously after a set time period has elapsed, or at a regular interval (e.g. a set number of times per second), talk about what they are useful for, and look at their inherent issues.

To understand asynchronous loops and intervals and what they are useful for.

Introduction

For a long time, the web platform has made available to JavaScript programmers a number of functions that allow you to asynchronously execute code after a certain time interval has elapsed, and to repeatedly execute a block of code asynchronously until you tell it to stop. These are:

The modern version of setInterval(); executes a specified block of code before the browser next repaints the display, allowing an animation to be run at a suitable framerate regardless of the environment it is being run in.

The asynchronous code set up by these functions actually runs on the main thread, but you are able to run other code between iterations to a more or less efficient degree, depending on how processor intensive these operations are. In any case, these functions are used for running constant animations and other background processing on a web site or application. In the following sections we will show you how they can be used.

setTimeout()

As we said before, setTimeout() executes a particular block of code once after a specified time has elapsed. It takes the following parameters:

A function to run, or a reference to function defined elsewhere.

A number representing the time interval in milliseconds (so 1000 equals one second) to wait before executing the code. If you specify a value of 0 (or omit this value altogether), the function will run immediately. More on why you might want to do this later.

Zero or more values that represent any parameters you want to pass to the function when it is run.

Note: Because timeout callbacks are executed cooperatively, there's no guarantee that they will be called after exactly the specified amount of time. Instead, they will be called after at least that much time has elapsed. Timeout handlers can't be run until the main thread reaches the point in its execution where it goes through these handlers to find the ones that need to be run.

The functions we specify don't have to be anonymous. We can give our function a name, and can even define it somewhere else and pass a function reference to the setTimeout(). The following two versions of our code snippet are equivalent to the first one:

That can be useful if we have a function that needs to be called both from a timeout and in response to an event, for example. But it can also just help keep your code tidy, especially if the timeout callback is more than a few lines of code.

setTimeout() returns an identifier value that can be used to refer to the timeout later, such as when you want to stop it. See Clearing timeouts below to learn how to do that.

Passing parameters to a setTimeout() function

Any parameters that we want to pass to the function being run inside the setTimeout() have to be passed to it as additional parameters at the end of the list. For example, we could refactor our previous function so that it will say hi to whatever person's name is passed to it:

function sayHi(who) {
alert('Hello ' + who + '!');
}

The name of the person to say hello to can then be passed into the setTimeout() call as a third parameter:

let myGreeting = setTimeout(sayHi, 2000, 'Mr. Universe');

Clearing timeouts

Finally, if a timeout has been created, you can cancel it before the specified time has elapsed by calling clearTimeout(), passing it the identifier of the setTimeout() call as a parameter. So to cancel our above timeout, you'd do this:

clearTimeout(myGreeting);

Note: See greeter-app.html for a slightly more involved demo that allows you to set the name of the person to say hello to in a form, and cancel the greeting using a separate button (see the source code also).

setInterval()

setTimeout() works perfectly when we need to run the code once after a set period of time. But what happens when we need to run the code over and over again, for example in the case of an animation?

This is where setInterval() comes in. This works in a very similar way to setTimeout(), except that the function you pass to it as the first parameter is executed repeatedly at no less than the number of milliseconds given by the second parameter apart, rather than at once. You can also pass any parameters required by the function being executed in as subsequent parameters of the setInterval() call.

Let's look at an example. The following function creates a new Date() object, extracts a time string out of it using toLocaleTimeString(), and then displays it in the UI. We then run the function once per second using setInterval(), creating the effect of a digital clock that updates once per second (see this live, and also see the source):

Just like setTimeout(), setInterval() returns an identifying value you can use later when you need to clear the interval.

Clearing intervals

setInterval() keeps running a task forever, unless we do something about it — we may well want a way to stop such tasks, otherwise we may end up getting errors when the browser can't complete any further versions of the task, or if the animation being handled by the task has finished. We can do this in the same sort of way as we stopped timeouts — by passing the identifier returned by the setInterval() call to the clearInterval() function:

Active learning: Creating your own stopwatch!

With this all said, we've got a challenge for you. Take a copy of our setInterval-clock.html example, and modify it to create your own simple stopwatch.

You need to display a time as before, but in this example, you need:

A "Start" button to start the stopwatch running.

A "Stop" button to pause/stop it.

A "Reset" button to reset the time back to 0.

The time display to show the number of seconds elapsed, rather than the actual time.

Here's a few hints for you:

You can structure and style the button markup however you like; just make sure you use semantic HTML, with hooks to allow you to grab the button references using JavaScript.

You probably want to create a variable that starts at 0, then increments by one every second using a constant loop.

It is easier to create this example without using a Date() object, like we've done in our version, but less accurate — you can't guarantee that the callback will fire after exactly 1000ms. A more accurate way would be to run startTime = Date.now() to get a timestamp of exactly when the user clicked the start button, and then do Date.now() - startTime to get the number of milliseconds after the start button was clicked.

You also want to calculate the number of hours, minutes, and seconds as separate values, and then show them together in a string after each loop iteration. From the second counter, you can work out each of these.

How would you calculate them? Have a think about it:

The number of seconds in an hour is 3600.

The number of minutes will be the amount of seconds left over when all of the hours have been removed, divided by 60.

The number of seconds will be the amount of seconds left over when all of the minutes have been removed.

You'll want to include a leading zero on your display values if the amount is less than 10, so it looks more like a traditional clock/watch.

To pause the stopwatch, you'll want to clear the interval. To reset it, you'll want to set the counter back to 0, clear the interval, and then immediately update the display.

You probably ought to disable the start button after pressing it once, and enable it again after you've stopped/reset it. Otherwise multiple presses of the start button will apply multiple setInterval()s to the clock, leading to wrong behavior.

Compare the above example to the following one — this uses setInterval() to accomplish the same effect:

let i = 1;
setInterval(function run() {
console.log(i);
i++
}, 100);

How do recursive setTimeout() and setInterval() differ?

The difference between the two versions of the above code is a subtle one.

Recursive setTimeout() guarantees the same delay between the executions, so for example 100ms in the above case. The code will run and then wait 100 milliseconds before it runs again, so the interval will be the same regardless of how long the code takes to run.

The example using setInterval() does things somewhat differently. The interval we choose includes the time taken to execute the code we want to run in. Let's say that the code takes 40 milliseconds to run — the interval then ends up being only 60 milliseconds.

When using setTimeout() recursively, each iteration can calculate a different delay before running the next iteration. In other words, the value of the second parameter can specify a different time in milliseconds to wait before running the code again.

When your code has the potential to take longer to run than the time interval you’ve assigned, it’s better to use recursive setTimeout() — this will keep the time interval constant between executions regardless of how long the code takes to execute, and you won't get errors.

Immediate timeouts

Using 0 as the value for setTimeout() schedules the execution of the specified callback function as soon as possible but only after the main code thread has been run.

For instance, the code below (see it live) outputs an alert containing "Hello", then an alert containing "World" as soon as you click OK on the first alert.

setTimeout(function() {
alert('World');
}, 0);
alert('Hello');

This can be useful in cases where you want to set a block of code to run as soon as all of the main thread has finished running — put it on the async event loop, so it will run straight afterwards.

Clearing with clearTimeout() or clearInterval()

clearTimeout() and clearInterval() both use the same list of entries to clear from. Interestingly enough, this means that you can use either method to clear a setTimeout() or setInterval().

For consistency, you should use clearTimeout() to clear setTimeout() entries and clearInterval() to clear setInterval() entries. This will help to avoid confusion.

requestAnimationFrame()

requestAnimationFrame() is a specialized looping function created for running animations efficiently in the browser. It is basically the modern version of setInterval() — it executes a specified block of code before the browser next repaints the display, allowing an animation to be run at a suitable frame rate regardless of the environment it is being run in.

It was created in response to perceived problems with setInterval(), which for example doesn't run at a frame rate optimized for the device, sometimes drops frames, continues to run even if the tab is not the active tab or the animation is scrolled off the page, etc. Read more about this on CreativeJS.

The idea is that you define a function in which your animation is updated (e.g. your sprites are moved, score is updated, data is refreshed, or whatever), then you call it to start the process off. At the end of the function block you call requestAnimationFrame() with the function reference passed as the parameter, and this instructs the browser to call the function again on the next display repaint. This is then run continuously, as we are calling requestAnimationFrame() recursively.

Note: If you want to perform some kind of simple constant DOM animation, CSS Animations are probably faster as they are calculated directly by the browser's internal code rather than JavaScript. If however you are doing something more complex and involving objects that are not directly accessible inside the DOM (such as 2D Canvas API or WebGL objects), requestAnimationFrame() is the better option in most cases.

How fast does your animation run?

The smoothness of your animation is directly dependent on your animation's frame rate and it is measured in frames per second (fps). The higher this number is, the smoother your animation will look, to a point.

Since most screens have a refresh rate of 60Hz, the fastest frame rate you can aim for is 60 frames per second (FPS) when working with web browsers. However, more frames means more processing, which can often cause stuttering and skipping — also known as dropping frames, or jank.

If you have a monitor with a 60Hz refresh rate and you want to achieve 60 FPS you have about 16.7 milliseconds (1000 / 60) to execute your animation code to render each frame. This is a reminder that we need to be mindful of the amount of code that we try to run for each pass through the animation loop.

requestAnimationFrame() always tries to get as close to this magic 60 FPS value as possible, although sometimes it isn't possible — if you have a really complex animation and you are running it on a slow computer, your frame rate will be less. requestAnimationFrame() will always do the best it can with what it has available.

How does requestAnimationFrame() differ from setInterval() and setTimeout()?

Let's talk a little bit more about how the requestAnimationFrame() method differs from the other methods we looked at earlier. Looking at our code from above:

As we said before, we don't specify a time interval for requestAnimationFrame(); it just runs it as quickly and smoothly as possible in the current conditions. The browser also doesn't waste time running it if the animation is offscreen for some reason, etc.

setInterval() on the other hand requires an interval to be specified. We arrived at our final value of 17 via the formula 1000 milliseconds / 60Hz, and then rounded it up. Rounding up is a good idea, as if you rounded down the browser might try to run the animation faster than 60fps, and it wouldn't make any difference to the smoothness of the animation anyway. As we said before, 60Hz is the standard refresh rate.

Including a timestamp

The actual callback passed to the requestAnimationFrame() function can be given a parameter too — a timestamp value that represents the time since the requestAnimationFrame() started running. This is useful as it allows you to run things at specific times and at a constant pace, regardless of how fast or slow your device might be. The general pattern you'd use looks something like this:

Browser support

requestAnimationFrame() is supported in slightly more recent browsers than setInterval()/setTimeout() — most interestingly it is available in Internet Explorer 10 and above. So unless you need to support older versions of IE with your code, there is little reason to not use requestAnimationFrame().

A simple example

Enough with the theory; let's go through and build our own requestAnimationFrame() example. We're going to create a simple "spinner animation", the kind you might see displayed in an app when it is busy connecting to the server, etc.

Note: In a real world example, you should probably use CSS animations to run this kind of simple animation. However, this kind of example is very useful to demonstrate requestAnimationFrame() usage, and you'd be more like to use this kind of technique when doing something more complex such as updating the display of a game on each frame.

Put an empty <div> element inside the <body>, then add a ↻ character inside it. This is a circular arrow character that will act as our spinner for this simple example.

Apply the following CSS to the HTML template in whatever way you prefer. This sets a red background on the page, sets the <body> height to 100% of the <html> height, and centers the <div> inside the <body>, horizontally and vertically.

Insert the following JavaScript inside your <script> element. Here we're storing a reference to the <div> inside a constant, setting a rotateCount variable to 0, setting an uninitialized variable that will later be used to contain a reference to the requestAnimationFrame() call, and setting a startTime variable to null, which will later be used to store the start time of the requestAnimationFrame().

Below the previous code, insert a draw() function that will be used to contain our animation code, which includes the timestamp parameter:

function draw(timestamp) {
}

Inside draw(), add the following lines. Here we define the start time if it is not defined already (this will only happen on the first loop iteration), and set the rotateCount to a value to rotate the spinner by (the current timestamp, take the starting timestamp, divided by three so it doesn't go too fast):

Below the previous line inside draw(), add the following block — this checks to see if the value of rotateCount is above 359 (e.g. 360, a full circle). If so, it sets the value to its modulo of 360 (i.e. the remainder left over when the value is divided by 360) so the circle animation can continue uninterrupted, at a sensible, low value. Note that this isn't strictly necessary, but it is easier to work with values of 0-359 degrees than values like "128000 degrees".

if (rotateCount > 359) {
rotateCount %= 360;
}

Next, below the previous block add the following line to actually rotate the spinner:

spinner.style.transform = 'rotate(' + rotateCount + 'deg)';

At the very bottom inside the draw() function, insert the following line. This is the key to the whole operation — we are setting the variable we defined earlier to an active requestAnimation() call that takes the draw() function as its parameter. This starts the animation off, constantly running the draw() function at a rate of as close to 60 FPS as possible.

Clearing a requestAnimationFrame() call

Clearing a requestAnimationFrame() call can be done by calling the corresponding cancelAnimationFrame() method (note, "cancel" not "clear" as with the "set..." methods), passing it the value returned by the requestAnimationFrame() call to cancel, which we stored in a variable called rAF:

cancelAnimationFrame(rAF);

Active learning: Starting and stopping our spinner

In this exercise, we'd like you to test out the cancelAnimationFrame() method by taking our previous example and updating it, adding an event listener to start and stop the spinner when the mouse is clicked anywhere on the page.

Some hints:

A click event handler can be added to most elements, including the document <body>. It makes more sense to put it on the <body> element if you want to maximize the clickable area — the event bubbles up to its child elements.

You'll want to add a tracking variable to check whether the spinner is spinning or not, clearing the animation frame if it is, and calling it again if it isn't.

Throttling a requestAnimationFrame() animation

One limitation of requestAnimationFrame() is that you can't choose your frame rate. This isn't a problem most of the time, as generally you want your animation to run as smoothly as possible, but what about when you want to create an old school, 8-bit-style animation?

This was a problem for example in the Monkey Island-inspired walking animation from our Drawing Graphics article:

In this example we have to animate both the position of the character on the screen, and the sprite being shown. There are only 6 frames in the sprite's animation; if we showed a different sprite frame for every frame displayed on the screen by requestAnimationFrame(), Guybrush would move his limbs too fast and the animation would look ridiculous. We therefore throttled the rate at which the sprite cycles its frames using the following code:

This is the code that works out how to update the position in each animation frame.

The method you use to throttle your animation will depend on your particular code. For example, in our spinner example we could make it appear to move slower by only increasing our rotateCount by one on each frame instead of two.

Active learning: a reaction game

For our final section of this article, we'll create a 2-player reaction game. Here we have two players, one of whom controls the game using the A key, and the other with the L key.

When the Start button is pressed, a spinner like the one we saw earlier is displayed for a random amount of time between 5 and 10 seconds. After that time, a message will appear saying "PLAYERS GO!!" — once this happens, the first player to press their control button will win the game.

Let's work through this.

First of all, download the starter file for the app — this contains the finished HTML structure and CSS styling, giving us a game board that shows the two players' information (as seen above), but with the spinner and results paragraph displayed on top of one another. We just have to write the JavaScript code.

Inside the empty <script> element on your page, start by adding the following lines of code that define some constants and variables we'll need in the rest of the code:

Next, below the previous lines of code, add the following function. This simply takes two numerical inputs and returns a random number between the two. We'll need this to generate a random timeout interval later on.

OK, enough preparation. Let's make the game playable! Add the following block to your code. The start() function calls draw() to start the spinner spinning and display it in the UI, hides the Start button so we can't mess up the game by starting it multiple times concurrently, and runs a setTimeout() call that runs a setEndgame() function after a random interval between 5 and 10 seconds has passed. We also add an event listener to our button to run the start() function when it is clicked.

Note: You'll see that in this example we are calling setTimeout() without storing the return value (so not let myTimeout = setTimeout(functionName, interval)). This works and is fine, as long as you don't need to clear your interval/timeout at any point. If you do, you'll need to save the returned identifier.

The net result of the previous code is that when the Start button is pressed, the spinner is shown and the players are made to wait a random amount of time before they are then asked to press their button. This last part is handled by the setEndgame() function, which we should define next.

First we cancel the spinner animation with cancelAnimationFrame() (it is always good to clean up unneeded processes), and hide the spinner container.

Next we display the results paragraph and set its text content to "PLAYERS GO!!" to signal to the players that they can now press their button to win.

We then attach a keydown event listener to our document — when any button is pressed down, the keyHandler() function is run.

Inside keyHandler(), we include the event object as a parameter (represented by e) — its key property contains the key that was just pressed, and we can use this to respond to specific key presses with specific actions.

We first log e.key to the console, which is a useful way of finding out the key value of different keys you are pressing.

When e.key is "a", we display a message to say that Player 1 won, and when e.key is "l", we display a message to say Player 2 won. Note that this will only work with lowercase a and l — if an uppercase A or L is submitted (the key plus Shift), it is counted as a different key.

Regardless of which one of the player control keys was pressed, we remove the keydown event listener using removeEventListener() so that once the winning press has happened, no more keyboard input is possible to mess up the final game result. We also use setTimeout() to call reset() after 5 seconds — as we explained earlier, this function resets the game back to its original state so that a new game can be started.

Conclusion

So that's it — all the essentials of async loops and intervals covered in one article. You'll find these methods useful in a lot of situations, but take care not to overuse them — since these still run on the main thread, heavy and intensive callbacks (especially those that manipulate the DOM) can really slow down a page if you're not careful.